In a radio frequency (RF) receiver, the signal received by an antenna usually is extremely small. For example, the RF signal received by an antenna for a TV signal may be as low as several micro-volts. The small received signal often is amplified by a low-noise amplifier (LNA) typically with low noise figure (NF) in order to ensure system performance in terms of signal to noise ratio (SNR). The LNA is one of the critical building blocks in an RF system since the RF signal received is amplified by the LNA and the performance of the LNA has a major impact on the system performance. While the LNA is required to amplify received RF signals which may have very small amplitude, the LNA may also encounter channels having a strong received signal. For these channels, the LNA only needs to provide a small degree of amplification or even to attenuate the signal; otherwise the amplified signal may be saturated. Furthermore, in a mobile environment, the received signal from a channel may fluctuate significantly depending on the distance between the transmitter and the receiver as well as other receiving conditions. Therefore, the LNA has to accommodate a wide range of input signal levels adaptively. Thus, LNA has to be designed to provide a wide range of gains without affecting its performance.
According to the above discussion, the LNA must be capable of providing a gain large enough to amplify a weak signal as well as a small gain or no gain for a strong signal at the LNA input. At the mean time, the LNA must have a low noise figure to amplify signal without introducing large distortion to ensure the signal quality. Typically, a larger gain is associated with a lower noise figure and a higher current for the current source used by the LNA circuit. The signal strength at the input of the LNA may vary for various reasons, such as performance of the antenna, the distance of the mobile device from the transmitter and the receiving environment. Therefore, the LNA design must provide a variable gain to adapt to the varying signal. A higher gain should be used for a smaller input signal and a lower gain should be used for a larger input signal. Usually an automatic gain control (AGC) is used to automatically set a proper gain for an input so that the amplified input signal will be within a range suited for subsequent processing. If the gain is not properly set, it may lead to under amplification, particularly for a weak input signal, which may affect the performance or operation of the system; or it may lead to amplifier saturation, particularly for a strong input signal which will cause signal distortion and degrade system performance. A conventional method of increasing the gain of the LNA is to increase the transconductance gm of the input device or the load resistance. Typical method of increasing the transconductance gm of the input device is to increase the bias current of the input device. Also, the transconductance gm can also be increased by simply increasing the size of the input device. Nevertheless, this would require multiple input devices with various sizes to implement various gains. Furthermore, the linearity will be sacrificed by this method. Either increasing the bias current or increasing the load resistance will increase the voltage across the load resistor. This implies that a higher operating voltage is required for the LNA, which is undesirable particularly for portable applications.
Both the noise figure and linearity are two important properties of the LNA besides the variable gain. A low noise figure is required for a weak signal at the LNA input; otherwise the noise from the LNA may become noticeable or even dominant in the amplified input signal. Good linearity is particularly important for the case of strong interference in the input signal; otherwise images associated with the interference signal may fall into the band of the intended signal. As described previously, increasing the size of the input device can increase the LNA gain. However, it will decrease the overdrive voltage of the input devices and consequently decrease the linearity of LNA. Increasing the bias current can improve both the noise figure and the linearity. However, the increased power dissipation due to higher bias current becomes a major concern for portable applications. Therefore, it always becomes a tradeoff between system performance and power/cost. It is desirable to design an LNA that provides a wide range of gains while maintaining high performance without the drawbacks of increased power consumption and/or cost.